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What is a Class?

In most object-oriented programming languages, a class is a user-defined type that includes a set of functions. As we have seen, structures in C++ meet the general definition of a class.

But there is another feature in C++ that also meets this definition; confusingly, it is called a class. In C++, a class is just a structure whose instance variables are private by default. For example, I could have written the Card definition:

class Card
{
  int suit, rank;

public:
  Card ();
  Card (int s, int r);

  int getRank () const { return rank; }
  int getSuit () const { return suit; }
  int setRank (int r) { rank = r; }
  int setSuit (int s) { suit = s; }
};

I replaced the word struct with the word class and removed the private: label. This result of the two definitions is exactly the same.

In fact, anything that can be written as a struct can also be written as a class, just by adding or removing labels. There is no real reason to choose one over the other, except that as a stylistic choice, most C++ programmers use class.

Also, it is common to refer to all user-defined types in C++ as "classes," regardless of whether they are defined as a struct or a class.
Home Classes and invariants What is a Class?

Last Update: 2005-12-05